Dental plaque represents one of the most complex microbial communities or biofilms known to afflict man. Oral biofilm-related diseases - dental carries, gingivitis and periodontitis - devastate a large human population and continue to pose a huge economic burden due to the lack of effective therapies. The long-term goal of this project is to elucidate the basic mechanisms of oral biofilm development and identify key players that may be attractive targets for developing drugs and vaccines. The development of dental plaque begins with the attachment of early bacterial colonizers to the tooth enamel, generating an adhesive matrix that then attracts the intermediate and late colonizers. Actinomyces is a key early colonizer that plays a prominent role in biofilm development by virtue of its ability to directly interact not only with the tooth surface but also with a number of both early and intermediate colonizers. Therefore, our studies have focused on dissecting the adhesive principles, i.e. fimbriae and other surface proteins, specifically involved in these interactions ad the mechanism of their assembly on the bacterial surface. During the past grant period, we succeeded in developing a facile new gene disruption technology for Actinomyces oris, and through it, identified the key components of two distinct fimbriae that are pivotal in the aforementioned cell-cell interactions. We showed that the tip fimbrillin FimQ serves dual functions, facilitating the assembly of type 1 fimbriae and directly mediating bacterial adherence to salivary proline-rich proteins known to coat the tooth surface. In contrast, the shaft fimbrilli FimA of the type 2 fimbriae mediates the receptor polysaccharide-dependent coaggregation with oral streptococci, adherence to erythrocytes and biofilm development. Structural studies revealed two adhesive IgG-like modules of FimA essential for its multivalent functions. Significantly, we showed that polymerization of these fimbrillins into fimbrial polymers requires their cognate fimbriae-specific sortase, a conserved transpeptidase in Gram-positive bacteria. The resulting polymers are anchored to the cell wall by the housekeeping sortase SrtA, which is also essential for the cell wall anchoring of many surface proteins with a cell wall sorting signal One of these, AcaF, is found to play a significant role in bacterial coaggregation. Most importantly, we discovered that inactivation of SrtA greatly perturbs bacterial morphology accompanied with abnormal cell wall and septa. Thus, the sortase machinery is a key player of bacterial pathogenesis and fitness in A. oris. Driven by these major advancements and several new hypotheses, the continuation proposal has three major aims: (1) Uncover the physiological function and regulation of sortase SrtA in cell surface homeostasis, (2) Dissect the molecular interactions of fimbrillins and non-fimbrial surface proteins with various cellular receptors, (3) Further elucidate the fundamental mechanisms of fimbrial assembly in Actinomyces, delineate the distinct mechanism of fimbrial assembly mediated by the tip fimbrillin FimQ, and identify trans- acting factors required for sortase-mediated fimbrial assembly.